Electrical energy today is primarily supplied through turbine based electrical generation. The electricity supply to the electrical grid must constantly be balanced against the accumulated electrical consumption and any losses in the grid. When there is unbalance, the frequency of the grid is affected such that an overproduction leads to an increase in frequency and an underproduction leads to a decrease in frequency in relation to the standardized frequency 50 Hz. Some balancing is done by allowing minor variations of the power frequency. If the deviation is too large, the grid is put out of order.
The production of electricity comprises two main parts, base power and load following power. The base power operates at a constant load and is produced by e.g. nuclear plants, coal or gas fired power plants, or combined heat and power plants (CHP). For stabilization of the frequency at 50 Hz momentary regulation (i.e. regulation of variations within seconds) of the rotating mass in the synchronic generators/turbines is of great importance.
For stabilization during longer periods, such as hours up to days, the electrical production and consumption is balanced by regulating the electrical consumption, by switching electrical consumers on or off from the grid, and/or by regulating the power production, i.e. by use of load following power plants such as hydro power plants in Sweden or gas fired power plants at the European continent.
By connecting different AC grids to each other, large multinational grids are created where the total rotating mass cooperates. However, electrical energy generated by solar or wind power has no rotating mass. Therefore, a certain amount of the electricity production still needs to be turbine based or include rotating equipment with no electrical generation. An estimate of the future energy mix in the Swedish grid, corresponding to an annual electrical generation of 55 TWh, is that 25% of the production needs to be turbine based while the remaining 75% can be based on wind power (45 TWh) and solar power (10 TWh).
The ongoing transition to renewable, intermittent electrical energy from wind and sun hence leads to completely different conditions for the energy supply since the electricity production depends on the weather, i.e. the variations are seasonal, daily, hourly, down to within seconds, and since the electrical energy generated by solar or wind power today normally has no rotating mass. In the future this stabilization is expected to be achieved by means of additional regulation of wind power turbines.
Also, any future overproduction of electricity can with use of today's technology be used or disposed of e.g. as waste, for export, or in district heating systems. Waste simply means that surplus energy is kept unused, i.e. that an amount of the solar or wind power plants are shut down in order to reduce the level of electricity production. Export means that surplus electricity is exported to another country. This is however not always possible since there has to be a market to which one can export the electricity. If the power supply in nearby countries, in the future, also develops into a similar wind and power supply, the possibilities for export will be limited. District heating systems means that an amount of the surplus electricity is transmitted into the district heating system by the use of a number of electric boilers. Since surplus exists only a short period of the year, the investment in electric boilers must be very efficient. Further, it is not possible to use all the generated electricity in the district heating system since the surplus production normally occurs during spring and summer at periods when the consumption is low, i.e. the production and the consumption is not balanced. These situations may also be a result of the difficulties in predicting both the weather and the electricity consumption.
Hence, there is a need for an improved energy storage system which utilizes the overproduction of electrical energy and which is used for evening out and regulating any variations in the electricity production, independently of it being seasonal, daily, hourly, or down to within seconds.